The exhaust gas discharged from an internal combustion engine, e.g., an automotive engine, contains harmful components such as particulate matter (PM), hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), and so forth. A wall-flow exhaust gas purification catalyst is used to efficiently remove these exhaust gas components.
A wall-flow exhaust gas purification catalyst are provided with an entrance cell having an open exhaust gas inflow-side end, an exit cell having an open exhaust gas outflow-side end, and a porous partition (rib wall) that partitions the two types of cells from each other. The exhaust gas discharged from the internal combustion engine flows into and within the entrance cell from the exhaust gas inflow-side end, passes through the pores of the porous partition, and flows out from the exhaust gas outflow-side end of the exit cell. The components of the exhaust gas are purified (detoxified) by contact between the exhaust gas and a catalyst layer (catalyst metal).
Patent Literature 1 and Patent Literature 2 are examples of prior art documents related to this. For example, Patent Literature 1 discloses an exhaust gas purification catalyst that is provided with a catalyst layer having a two-layer structure. Specifically, an exhaust gas purification catalyst is disclosed that is provided with a Pd-containing first catalyst layer over the entire interior part of the partition and that is provided with an Rh-containing second catalyst layer on the surface of the partition on the side in contact with the entrance cell so as to completely coat the first catalyst layer.